The herbicidal activity of 2-(4-aryloxyphenoxy)propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Pat. No. 4,531,969 discloses that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are required to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages. Analogous advantages exist in the pharmaceutical art as exemplified by the anti-inflammatory 2-(phenyl)propionic acids such as ibuprofen.
In order to commercially exploit the benefits of the advantages associated with the use of the biologically more active enantiomer, it is necessary to accurately determine the ratio of the R- and S-enantiomers in a specific composition. Besides allowing one to monitor the production process, such analytical capability is required to support product labeling and registration requirements. One of the most versatile methods of achieving such an analysis involves the synthesis of diastereomeric derivatives. By reacting a racemate or a partially resolved mixture of enantiomers with an optically active reagent, a pair of diastereomeric derivatives are produced which can be separated on the basis of their different physical properties.
For carboxylic acids in general, including the 2-(phenyl or phenoxy)propionic acids, optically active amines such as .alpha.-methyl benzyl amine have found widespread use as the resolving agents of choice. Oftentimes, however, the chromatographic separations of the pair of diastereomers is less than ideal. Furthermore, quantification is not always accurate because of the purity of the commercial samples available or because of slight racemization during the derivatization reaction. This proves particularly troublesome where accurate determinations of relatively small amounts of one diastereomer in the presence of relatively large amounts of the other are desired.